1. Field of the Invention
The invention relates to an optical instrument for measuring the speed or length of a moving surface, in which a measuring light beam from a laser is directed onto the surface and the Doppler-shifted stray light reflected by the surface and other light, e.g. non-Doppler-shifted laser light or some other Doppler-shifted laser light are superimposed for obtaining a beat, whose frequency forms a measure for the speed of the surface and is received and evaluated in the form of continuously supplied signals in an evaluation system and is in particular counted in a counting system.
2. Prior Art
The optical part of known instruments normally supplies a small, very noisy, intermittent signal in the HF range to the evaluation system, in which the signal must initially be filtered, so as to separate the signal component from the noise component. In order to reduce the noise band width a reduction of the overall band width of a noisy signal is sought, which makes it possible to improve the signal-to-noise (S/N) ratio, for which purpose a band pass filter is used. As the frequency of the continuously supplied signals is not constant, the centre frequency of the band pass filter must be correspondingly followed. Therefore a high selectivity band pass filter with variable centre frequency is required.
Alternatively, preference is frequently given to a highly selective crystal band pass filter with a stationary centre frequency. In order to be able to filter a variable frequency signal with such a band pass filter, either the band pass centre frequency must be moved over the spectrum, or the spectrum must be moved past a fixed band pass centre frequency. The moving of the spectrum past a fixed band-pass centre frequency is known as the heterodyne process. However, at the output, it supplies an output frequency, which is the same as the centre frequency of the band pass filter. If the control voltage of the frequency generator VCO is measured and evaluated as the output signal, the desired precision is not obtained with this system.